Sewage sludges particularly from community sewage clarification plants (digested sludge, raw sludge) generally contain more than 90% water. For economic disposal - such as dumping or incineration - sewage sludges are dewatered using suitable dewatering plants. Dewatered sludge will contain only 75% to 50% water and in this condition is referred to as "disposable". Suitable dewatering equipment includes band filter presses, centrifuges and recessed plate pressing apparatus.
The dewatering of sewage sludges calls for conditioning with dewatering aids. Where dewatering is carried out using centrifuges or band filter presses, it has been common practice for decades to add organic polyelectrolytes to act as flocculating agents.
For dewatering in recessed plate pressing equipment, it has been common practice in the past to use inexpensive inorganic additives such as ferrous salts and lime. The amounts required for such additives are so great that in many cases the conditioning agents make up more than 50% of the filter cake solids, resulting in a substantial increase in transportation and disposal costs. It has been known for some time now that organic polyelectrolytes may form pressure-stable solids agglomerates that dewater readily in centrifuges, band filter presses and in recessed plate equipment.
Apart from lower costs, the advantages of using organic conditioning flocculants include the absence of undesirable constituents to be disposed of together with the filter cake, reduced capital investment, and the possibility of incinerating dewatered sewage sludge from recessed plate pressing equipment.
In general, the sludge to be dewatered is stored in thickening containers, transported to the recessed plate press building and then conditioned and dewatered. In conventional conditioning using inorganic additives such as lime and ferrous salts, the sludge is agitated batch-wise in suitable conditioning containers and then pumped into the recessed plate pressing equipment preferably by piston-diaphragm-type pumps.
In the past, sludge conditioning using organic polymers was carried out in a similar manner, with the amount of flocculant added proportional to solids content (German Pat. No. 33 46 834). Preferably, the flocculant solution was added on the delivery side of the sludge pump so as to avoid mechanical floc destruction in the sludge pump. In-line addition on the delivery side of the feed pumps is hard to cope with, however, as the great variance of sludge throughput and flow rates between the starting phase of a batch (e.g. 40 cu.m. of sludge per hour) and its final or high pressure phase (e.g. 4 cu.m. of sludge per hour) renders optimum flocculation and admixture exceedingly difficult. In general, the flocculant solution has to be added in a highly dilute form (0.1%-0.5% solution) so that hydraulic loading on the recessed plate press and pressing times in polymer conditioning will increase.
Attempts have been made to avoid this disadvantage by a preliminary water separation treatment. In the past, this approach has required the use of mechanical dewatering units (revolving screens); cf. DE-As specification No. 29 20 350. A disadvantage of the prior process is the equipment of using additional machinery and additional energy consumers.
In work which contributed to the present invention, it has been shown that the sludge so flocculated and mechanically loaded in the additional dewatering unit had to be reactivated so as to enable the subsequent dewatering treatment to be carried out in an efficient manner.
The object underlying the present invention is to provide a method of dewatering sewage sludge which decreases hydraulic loading in pressure filtration, reduces dewatering times and enhances filtering performance without calling for additional equipment such as preparatory dewatering units.
At the same time, the invention seeks to provide high solids filter cakes which come off readily and by themselves from the filter cloth used in recessed plate pressing installations.